A protein specific to mitochondria from S-type male-sterile cytoplasm of maize is encoded by an episomal DNA.

Mitochondria of S-type cytoplasmic male sterile maize contain two linear double-stranded DNA molecules, S1 and S2. Two open reading frames (ORF1 and ORF2) are present in S2 DNA. Fragments from ORF1 were inserted into plasmids to achieve expression in Escherichia coli. Cells transformed with recombinant plasmids produced mRNA which hybridized with ORF1 and corresponding polypeptides were synthesized by in vivo and in vitro systems. Antiserum against a lacZ/S2 fusion protein precipitated the anticipated polypeptides from transformed E. coli cells and was therefore used to detect homologous peptide sequences in protein preparations of mitochondria from different maize cytoplasms. The antiserum detected a protein of 125 000 M(r) present in mitochondria from male sterile B73S but absent from the fertile B73N cytoplasm.